Age-related physiological changes predispose to heart failure with preserved ejection fraction (HFpEF). Thus, HFpEF prevalence is escalating as the older population expands. High mortality and morbidity, diminished quality of life, and spiraling healthcare costs are typical consequences, and no effective HFpEF therapy is known. Therefore, several small exercise training (ExT) trials for HFpEF stand out by showing that ExT result in improved aerobic exercise capacity and infer that ExT constitutes novel substantive therapy. Nonetheless, such benefit was evident only after months of moderate to high intensity ExT; regimens that are unfeasible for most patients. In fact, poor compliance with ExT is typical in most HFpEF patients. We propose there are intrinsic physiological components of HFpEF pathophysiology that predispose to ?fatigability?. We advance the concept of fatigability by quantifying it as a performance-based measure; i.e., subjective tiring during a standardized steady-state walking (perceived fatigability) and deterioration of self-selected walking speed over time (performance fatigability). We assert that therapies to reduce fatigability will enhance HFpEF outcomes. Ongoing studies reveal pleotropic benefits of oral inorganic nitrite (NO2), including enhanced performance of skeletal muscle (metabolism and bioenergetics) and vasomotor responses (systemic and pulmonary). Our pilot work shows safety and biological efficacy of oral NO2 capsules. Thus, we propose a randomized, controlled, double-blinded trial to study oral NO2 therapy in older (?70 years) HFpEF patients. Aim 1 explores the utility of NO2 capsules to reduce perceived and performance fatigability (rated perceived exertion), improve aerobic capacity (peak oxygen uptake) and increase daily activity (accelerometry). Aim 2 delineates the mediating processes by which NO2 benefits are achieved. Skeletal muscle determinants are differentiated from the right and left heart vasomotor dynamics by integrating assessments using 31P magnetic resonance spectroscopy and percutaneous needle muscle biopsies with those made using non-invasive and invasive cardiopulmonary exercise testing, near infrared spectroscopy and other techniques. The principal investigator is trained geriatrics and cardiology, and is solidly oriented to the dynamics of aging and cardiovascular disease (clinically and mechanistically) with particular expertise in functional assessment and skeletal muscle gene expression as determinants of performance. The investigative team provides formidable synergies that are well-suited to this translational investigation of systemic, cellular, and sub-cellular physiological dynamics. Our proposal is significant in multiple respects: 1) HFpEF is endemic with aging and constitutes a critical contemporary healthcare challenge today?s growing population of older adults. 2) Fatigability is rooted in HFpEF pathophysiology, but it has not previously been addressed as a key part of management. 3) NO2 therapy is a novel and compelling therapeutic strategy. 4) Mechanisms underlying fatigability are clarified; we advance principles of patient-centered care by clarifying mechanisms that underlie a patient?s experience of fatigability.